Doubts in Salaah

Q: Is it permissible/recommended for a person to spit in salaah if he finds whispers of shaitaan troubling him? Is there anything else he should rather do based on authentic narrations? 

The below narration is from Muslim Shareef: 

O Messenger of Allah, the shaytaan interferes between me and my prayer and my recitation, and he makes me confused. The Messenger of Allah (blessings and peace of Allah be upon him) said: “That is a devil called Khinzab. If you feel that, then seek refuge with Allah from him and spit dryly to your left three times.” He said: I did that and Allah took him away from me. 

Please advise.

Q: What should I do if I forget how many rakʿāt I’ve performed during prayer?

Q: If one prayed Maghrib and though it was the second unit so he stood up from Tashahhud and then he had a doubt and thought actually he has already prayed three and this is a fourth but he went with the first thought and completed that unit even though the thought that this was a fourth unit was strong and he did Sajdah Sahw, would it be Wajib to repeat the prayer?

Q: If one is unsure if one prostrated once or twice in a unit but learns that it was twice, does one have to do sajda sahw at the end of the prayer?

If one does not do Sajdah Sahw is the prayer valid?

Q: Most of the time, I feel during Salah, I didn't do sujood or didn't recite Surah Fatiha, or didn't say tashahhud or didn't pray complete rakaats so I most of the time do Sajda Sahw. Am I doing it correct?

Q: What should a person do if he doubts (during salah but after the posture) whether he had read a waajib such as the duaa e qunoot?

Q: I am reading my salaah alone and I am not sure whether I read 1 rakaat or 2 rakaats. Should I make sajida sahw or break the salaah and restart again.

Q: In the witr prayer, if a person has a doubt as to whether they are performing the first rakat or the second rakat and this doubt occurs quite often and because their heart is not inclined towards either rakat that person chooses the lesser number and performs the first rakat along with the tashahhud because it is possible that this could be the second rakat. In the second rakat does this person have to recite the qunoot because it is possible that this could be the third rakat? In the third rakat, does this person have to recite the qunoot again along with prostration of forgetfulness.

Q: I was reciting tashahud and I had to clear my throat and I wasnt sure if I had begun reading assalamu alaika ayyuhan nabiyyu warahmatullahi wabarakatu or if I actually already did read it and I should start reading assalamu alayna. I decided to continue by reciting assalamu alayna. Does this doubt render my salah invalid?

Q: I was performing the Farz of Zuhr. I kinda forgot about the prayer and my mind was wandering about my result which will come in a few days. So I don't remember if I offered 4 farz or just said the salam after 2. What should I do when I have a doubt ?